1. Field of the Invention
The present invention relates in general to an organic electroluminescent(EL) material. In particular, the present invention relates to a high efficiency blue and green organic EL material.
2. Description of the Related Art
An Organic Light Emitting Diode (OLED) with high efficiency was reported by C. W. Tang and S. A. Vanslyke in 1987. Device improvements incorporating fluorescent dyes in the emitter layers were demonstrated in 1989. Since then, flat panel display development based on OLEDs, has accelerated rapidly. In recent years, several laboratories around the world have been engaged in developing the materials, devices and processes needed to bringing this technology to commercialization.
FIG. 1 shows a schematic diagram of the conventional single layer OLED device structure. The substrate 8 is an electrically insulating and optically transparent material such as glass or plastic. Anode 6 is located on the substrate 8 and separated from cathode 2 by an organic EL medium 4. The anode 6 and the cathode 2 are connected to an external AC or DC power source 5. In operation, the device can be viewed as a diode that is forward biased when the anode 6 is at a higher potential then the cathode 2. Under these conditions, holes (positive charge carriers) and electrons are injected from the anode 6 and the cathode 2 into the organic EL medium 4, respectively. This results in hole-electron recombination and a release of energy in part as light, thus producing electroluminescence.
FIG. 2 is a schematic diagram of the conventional double layer OLED device structure. The substrate 20 is an electrically insulating and optically transparent material such as glass or plastic. Anode 18 is located on the substrate 20 and separated from cathode 12 by an electron-transport layer 14 and a hole-transport layer 16. The hole-transport layer 16 is formed on the anode 18. Located above the hole-transport layer 16 is the electron-transport layer 14. The anode 6 and the cathode 2 are connected to an external AC or DC power source 15. In the double layer OLED device structure, the hole-transport layer 16 is specifically chosen to inject and transport holes, and the electron-transport layer 14 is specifically chosen to inject and transport electrons. The interface between the two layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. Moreover, the double layer structure has been developed to multilayer structure, and the multilayer structure including electron-inject layer, electron-transport layer, hole-inject layer, hole-transport layer, and emissive layer, etc.
Typical organic emitting materials were formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. However, for the production of full color OLED display panel, it is necessary to have efficient red, green and blue (RGB) EL materials with proper chromaticity and sufficient luminance efficiency. The guest-host doped system offers a ready avenue for achieving such an objective, mainly because doping an emissive dopant (guest) of high luminescent property into a host can raise the efficiency of radiative recombination. Therefore, it is very important to open up new and efficient organic EL materials.
An object of the present invention is to provide an organic electroluminescent compound of the formula (I): 
wherein:
X1 is selected from oxygen, sulfur, C(CH3)2 and Nxe2x80x94R, wherein R is hydrogen, alkyl of from 1 to 20 carbon atoms or aryl; and
Ar1 and Ar2 are individually aryl or heterocyclic systems.
Ar1 is shown as: 
wherein:
R1, R2, R3 and R4 are individually selected from the group consisting of hydrogen, halogen, substituted or unsubstituted alkyl group, substituted or unsubstituted alkoxy group, substituted or unsubstituted thioalkyl group, substituted or unsubstituted arylene group, substituted or unsubstituted aryloxy group, substituted or unsubstituted arylthio group, substituted or unsubstituted arylamino group, substituted or unsubstituted carbocyclic aromatic group, substituted or unsubstituted heterocyclic aromatic group, nitro group, and cyano group.
Ar2 is selected from the group consisting of substituted or unsubstituted biphenylene, substituted or unsubstituted triphenylene, substituted or unsubstituted terephenylene, substituted or unsubstituted bithiophene, substituted or unsubstituted trithiophene, substituted or unsubstituted terephenylene, substituted or unsubstituted arylene vinylene, substituted or unsubstituted carbazole, substituted or unsubstituted arylamino group, substituted or unsubstituted carbocyclic aromatic group, and substituted or unsubstituted heterocyclic aromatic group.
Another object of the present invention is to provide an organic electroluminescent compound of the formula (II): 
wherein:
X2 and X3 are individually selected from the group consisting of oxygen, sulfur, C(CH3)2 and Nxe2x80x94R, wherein R is hydrogen, alkyl of from 1 to 20 carbon atoms or aryl; and
Ar1, Ar2 and Ar3 are individually aryl or heterocyclic systems.
Ar1 and Ar3 are shown as: 
wherein:
R1, R2, R3 and R4 are individually selected from the group consisting of hydrogen, halogen, substituted or unsubstituted alkyl group, substituted or unsubstituted alkoxy group, substituted or unsubstituted thioalkyl group, substituted or unsubstituted arylene group, substituted or unsubstituted aryloxy group, substituted or unsubstituted arylthio group, substituted or unsubstituted arylamino group, substituted or unsubstituted carbocyclic aromatic group, substituted or unsubstituted heterocyclic aromatic group, nitro group, and cyano group.
Ar2 is selected from the group consisting of substituted or unsubstituted biphenylene, substituted or unsubstituted triphenylene, substituted or unsubstituted terephenylene, substituted or unsubstituted bithiophene, substituted or unsubstituted trithiophene, substituted or unsubstituted terephenylene, substituted or unsubstituted arylene vinylene, substituted or unsubstituted carbazole, substituted or unsubstituted arylamino group, substituted or unsubstituted carbocyclic aromatic group, and substituted or unsubstituted heterocyclic aromatic group.
Applying the above-mentioned compounds of the present invention to an EL device provides a highly efficient blue and green emitting organic EL device.